The present invention generally relates to A.C./D.C. converters including a rectifying element and a filtering element, and more specifically to the protection of loads of such converters against current peaks.
FIG. 1 schematically illustrates by functional blocks a conventional example of such a converter. A source 1 of a high A.C. voltage Vac supplies a load 2 via a rectifying means 3 and a filtering capacitor CF. Capacitor CF and load 2 are connected in parallel between two rectified output terminals of rectifying means 3, and two A.C. input terminals are connected across source 1.
The embodiments of the present invention will be discussed in relation with a rectifying means 3 of fullwave type. However, the present invention also applies to rectifications of halfwave type or others for which the principles and problems discussed hereafter are also encountered.
The operation of the converter of FIG. 1 is the following. As long as voltage Vac provided by source 1 is greaterxe2x80x94in absolute valuexe2x80x94than the rectified and filtered voltage VL across capacitor CF, source 1 supplies load 2 and capacitor CF charges. As soon as voltage Vac becomes smaller than voltage VL, rectifying means 3 is invalidated, generally automatically, and isolates its A.C. input terminals from its rectified output terminals. Load 2 is then supplied by the discharge of capacitor CF until the supply voltage Vac again becomes greater than the rectified and filtered voltage VL.
A disadvantage of such conventional A.C./D.C. converters is the transmission, by rectifying means 3, of possible current peaks appear, for example, upon the first powering-on or after microfailures of the power supply. xe2x80x9cMicrofailurexe2x80x9d is used to designate supply interruptions of a duration such that capacitor CF discharges to a level much lower than the mean steady state level. Since the converter generally is sized for a released power adapted to the load, such a phenomenon typically occurs for failures lasting for more than one period of the A.C. power supply (for example, 20 ms for a 50-Hz power supply).
To overcome this disadvantage, various approaches have been provided.
A first approach consists of sizing capacitor CF and load 2 according to such current peaks. This is not desirable, since it imposes the use of relatively bulky components. Further, this is not possible with all loads.
A more recent approach typically provides, as illustrated by dotted lines in FIG. 1, a device 4 of current limitation between source 1 and rectifying means 3. Device 4 can be made in several ways. Generally, it is a resistor with a negative temperature coefficient, or NTC resistor. Such an NTC resistor has, when cold, a very high resistance. Under the effect of the current flowing therethrough, it heats up and its resistance decreases. Such a device thus enables efficient reduction of the transmission of current peaks upon the first circuit power-on. However, in case of an incidental interruption of the power supply for a very short duration, the NTC resistor does not have time to cool down and cannot accordingly limit the current when the power supply reappears.
To solve this problem, limiting devices adapted to avoiding the transmission of current peaks after a microfailure have been developed. A first solution consists of replacing the NCT resistor with a fixed resistor in parallel with a controllable switch such as a triac or a relay. In steady state, with a substantially constant current, the triac or relay short-circuits the resistor. In transient phases, the triac or relay is open and the current flowing to capacitor CF and load 2 is limited by the fixed resistor. However, such devices have disadvantages in terms of response rapidity, increase of the power dissipation, and bulk. For example, a relay device imposes a relatively complex and highly dissipative control circuit and uses signals external to the device or to the converter, as well as an auxiliary low-voltage power supply.
It has also been provided to charge capacitor CF progressively by using a phase angle charge circuit. Such a circuit enables only triggering the capacitor charge when voltage Vac is greater than voltage VL by a determined threshold, generally of a few tens of volts. This relatively efficient system however also has implementation disadvantages, especially in terms of bulk. Further, like a relay device, it requires an additional control circuit and low-voltage power supply.
The embodiments of the present invention provide a novel A.C./D.C. converter that is directed to avoiding the transmission of current peaks to the load.
The converter has a reduced power loss, and in one embodiment an autonomous converter is provided, that is, does not imply use to an additional power supply. In accordance with another aspect of the present invention, an automatic converter is provided that does not require the use of signals external to the very converter. In accordance with another aspect of the present invention, a converter that automatically adapts to the amplitude of the supply voltage is provided.
The embodiments of the present invention provide an A.C./D.C. converter including a filtering capacitor, a first branch essentially including a first rectifier and a current limiter; a second branch essentially including a second rectifier, the series voltage drop of which is limited to that of the switches forming it; and a selection circuit for selecting one of the two rectifiers.
According to another embodiment of the invention, the selection circuit is adapted to select the second branch when the interval between the voltage across the filtering capacitor and the maximum value of the supply voltage is smaller than a determined reference value.
According to another embodiment of the present invention, the first branch includes a delayer of the turning-on of the first rectifier with respect to the zero crossing of the interval between the supply voltage and the rectified and filtered voltage, to enable selection of the second branch by the selection circuit.
According to yet another embodiment of the invention, the selection circuit includes a control circuit and a selection switch interposed between an output of the first rectifier and a terminal of control of at least one switch associated with the second branch.
According to a further embodiment of the invention, the control circuit includes a peak detector of the maximum level of the non-attenuated supply voltage, and a resistive divider adapted to determining the determined reference value.
According to still yet another embodiment of the invention, the switch of the second branch is a one-way switch constitutive of the second rectifier.
According to a further embodiment of the invention, the converter is of fullwave type, the first rectifier is a diode bridge and the second rectifier being a composite bridge, the two branches of which each include a diode and a controllable switch, the two diodes of the second rectifier being recirculation diodes shared with the first rectifier.
According to another embodiment of the invention, the delayer is a cathode-gate thyristor, the anode of which is connected to an output terminal of the current limiter, the cathode of which is connected to a terminal of a filtering capacitor, and the gate of which is brought back to the anode via a zener diode having its anode connected to the cathode gate of a thyristor.
According to yet a further embodiment of the invention, the selection switch is formed of a first transistor having a first power terminal, connected to an output terminal of the limiter, which forms a first input terminal of the selection circuit, a second power terminal connected to control terminals of input or output switches of the second rectifier, which forms the output of the selection circuit, and a control terminal connected to a first power terminal of a second transistor of the same type as the first transistor, a second power terminal of the second transistor connected to the output of the first rectifier forming a second input terminal of the selection means and a control terminal of the second transistor forming the control terminal of the switch receiving a control signal from the control circuit.
According to still yet another embodiment of the invention, the current limiter is connected between an output terminal of the first rectifier and a first rectified voltage terminal of the filtering capacitor.